Since fossil fuels are non-renewable resources, adequate supplies of energy and organic feedstocks need to be secured for the future. A transition to sustainable resources requires new technologies for the construction of improved feedstocks, the design of efficient processes to convert the feedstocks into valuable products, and/or the design of products that efficiently utilize an altered substrate spectrum. This transformation will create benefits such as decreased pollution from energy production and use, decreased pollution from chemical manufacturing processes, increased sustainability through the utilization of renewable natural resources and organic waste products as substrates, decreased dependence on foreign country's raw materials, and an increase in local economies and markets involved in the production of new substrates.
Plant biomass is one sustainable resource that can help meet future feedstock requirements. The use of plants as substrates for energy, chemical, pharmaceutical, and organic feedstock takes advantage of existing large-scale agricultural production, uses energy from the sun to incorporate carbon dioxide into plants via photosynthesis, and has fewer environmentally hazardous by-products. By using photosynthesis, plants make the carbon dioxide removed from the air available for the production of energy, chemicals, and agricultural products. Finding ways to effectively redistribute this carbon in forms that are readily and economically employable remains a challenge.
The production of chemical feedstocks and fuels from plant biomass is still in its infancy. Starch-based raw materials, for example, may be applied to the production of commodity or specialty chemical products. Poor substrate and strain availability hampering bioconversion, along with real or perceived safety issues related to containment, and a lack of economic viability, have made progress in this area particularly slow. Non-cellulosic biomass, such as corn starch, compares favorably with fossil resources on a mass basis, but is too costly. Cellulosic biomass, such as short-rotation poplar, pine, switchgrass, corn stover, sugar cane bagasse, waste paper sludge, and municipal solid waste, in contrast, is cost competitive in terms of both mass and energy. Cellulosic biomass, because of its complex structure, is nevertheless difficult to process. Currently, cellulosic biomass requires pretreatment with strong acids, bases, and/or other chemicals for use as a substrate for fuel, e.g. ethanol, or for chemical production, e.g. paper products. This pretreatment efficiently exposes polymeric subunits, primarily hexoses, pentoses, and phenolic compounds, which are then cleaved and used as substrates, but is expensive. One alternative to the use of more hazardous chemicals is the use of enzymes, although it is less cost effective.
Recombinant DNA technology has been applied to alter microorganisms to perform substrate bioconversion at reduced costs, thus expanding the use of microorganisms, and increasing the number of products that are produced. For example, plant cells that express lignocellulosic degrading enzymes have been constructed, although they rarely differentiate and regenerate into complete plants due to decomposition of structural components. In cases where they differentiate into complete plants, e.g. with lignin and cellulose substrates, the enzyme activities are low and the plants require further processing. Attempts to combine pretreatment of substrate biomass with fermentation have encountered difficulties as well, in part because of mass transfer limitations and interference with the fermenting organism.
CIVPS or inteins are in-frame, self-cleaving peptides that generally occur as part of a larger precursor protein molecule. CIVPS or inteins differ from other proteases or zymogens in several fundamental ways. Unlike proteases that cleave themselves or other proteins into multiple, unligated polypeptides, CIVPS or inteins have the ability to both cleave and ligate in either cis or trans conformations. Thus as opposed to terminal cleavage that would result from the reaction of a protease on a protein, CIVPS or inteins have the ability to cleave at multiple sites, and ligate the resulting protein fragments. This cleavage is induced under specific conditions and can be engineered using molecular biology techniques. CIVPS or inteins have been described in the literature in Saccharomyces cerevisiae (Kane et. al., Science 250:651; Hirata et al., J. Bio. Chem. 265:6726 (1990)), Mycobacterium tuberculosis (Davis et al., J. Bact. 173:5653 (1991), Davis et al., Cell 71:1 (1992)), Thermococcus litoralis (Perler, et al., PNAS 89:5577 (1992)), and in other organisms, but do not occur naturally in plants.
Accordingly, there is a need for providing novel methods for producing energy and other pharmaceutical or industrial products from more easily renewable sources, such as by modifying plants in a manner such that they may be used as energy and chemical feedstocks.